1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser device which emits a laser beam in a vertical direction by a resonator extending in the vertical direction.
2. Description of the Related Art
In general, an end face emitting semiconductor laser device has a large difference in gain between the vertical direction and the horizontal direction with respect to an epitaxial layer in a light wave-guiding channel, and therefore has stable polarization characteristics. On the other hand, a surface emitting semiconductor laser device called VSSEL (Vertical Cavity Surface Emitting Laser), as for example shown in FIG. 17, has a resonator 97 set in the vertical direction relative to a substrate 91, and a current injected through a p-type electrode 95 and an n-type electrode 96 causes an active layer 93 to emit light. The light is amplified by semiconductor multilayer reflective layers having a reflectance of approximately 100% which are called DBR (Distributed Bragg Reflector) mirrors (a lower DBR layer 92 and an upper DBR layer 93), and a laser beam L9 is made to go out through a beam outgoing aperture 98. The surface emitting semiconductor laser device 9 with such a configuration does not have any anisotropy in gain in the plane vertical to the outgoing direction of the laser beam L9. In its polarization characteristics, therefore, the surface emitting semiconductor laser device 9 has an ununiformity in that dispersion of the direction of polarization would be generated due to dispersion of the device, and an instability in that the direction of polarization would be easily varied depending on output and ambient temperature.
Therefore, in the case of applying such a surface emitting semiconductor laser device to a polarization-dependent optical device such as mirror and beam splitter, i.e., in the case where the surface emitting semiconductor laser device is used, for example, as a light source in a digital copying machine or printer, the dispersion of the direction of polarization would generate differences in the image forming position or in the output, leading to blurring or irregularities in color.
In view of this, there have been proposed several technologies for stabilizing the polarization direction into one direction by providing a polarization controlling function in the inside of a surface emitting semiconductor laser device.
In one example of such technologies, use is made of a special inclined substrate having the (311) plane oriented in the normal and including gallium-arsenic (GaAs). Where a surface emitting semiconductor laser device is fabricated by use of such a special inclined substrate, the gain characteristic with respect to the [−2 33] direction is enhanced, and the polarization direction of the laser beam can be controlled to this direction. In addition, the polarization ratio of the laser beam is very high. Thus, this technology is effective in controlling the polarization in a surface emitting semiconductor laser device.
In another example of the above-mentioned technologies, a resonator oriented in the vertical direction is so shaped as to have an in-plane anisotropy, thereby controlling the direction of polarization of a laser beam. For example, Japanese Patent No. 2891133 discloses a technology in which the post shape of a resonator is reduced into such a region that light undergoes a diffraction loss and is made to be rectangular (with a minor side of not more than 5 μm and a major side of 6 μm), whereby the polarization direction is controlled to the longitudinal direction in which the diffraction loss is smaller. Where the post shape of the resonator is configured in this manner, the direction of polarization of the laser beam can be set to an arbitrary direction by forming the minor side and the major side in arbitrary orientations.
Furthermore, JP-A-2001-525995 discloses a technology in which a discontinuity part is formed at a part of a metallic contact layer such as not to influence the characteristics of the laser beam made to go out through a beam outgoing aperture, to thereby obtain a polarized beam in a direction parallel to the boundary of the discontinuity part.